There is known from Japanese Patent Application Laid-open No. 2001-207908 an arrangement in which a cooling water passage is formed in a cylinder head of an internal combustion engine so as to cover an upper part of a combustion chamber. A large amount of cooling water is made to flow through a section of the cooling water passage where there is a large thermal load and a small amount of cooling water is made to flow in a section where there is a small thermal load so that the cylinder head is cooled to the minimum necessary; as a result, the temperature of exhaust gas is maintained as high as possible, thereby promoting warm-up of the internal combustion engine, and giving early activation of an exhaust gas purification catalyst.
In order to recover as much as possible of the waste heat of an internal combustion engine as high temperature steam, it is not sufficient just to make an evaporator provided in an exhaust passage exhibit its full performance by maintaining the exhaust gas at a high temperature; it is also necessary to recover heat that is transferred from the combustion chamber to the cylinder head and dissipated into the air. The heat that is transferred from the combustion chamber to the cylinder head therefore needs to be recovered effectively by water flowing through a cooling water passage formed around the combustion chamber of the cylinder head, thus increasing the temperature of the cooling water as much as possible.